Degradation Mechanism of Flexural Performance of GFRP—Steel Hybrid-Reinforced Concrete Beams under Coupled Composite Salt Corrosion and Sustained Loading

This study investigates the degradation of the flexural performance of reinforced concrete beams, GFRP-reinforced concrete beams, and GFRP-steel hybrid-reinforced concrete beams exposed to coupled composite salt wetting-drying cycles and sustained loading. A total of 39 beams were tested under four-point bending, with the main variables including reinforcement configuration, number of wetting-drying cycles, exposure temperature, and sustained loading condition. The results show that pre-existing flexural cracks induced by sustained loading provide preferential pathways for ion ingress, thereby accelerating concrete micro cracking, steel corrosion, and interfacial bond deterioration. Under coupled exposure, all beam types exhibited pronounced stiffness degradation and load capacity loss. Owing to the corrosion resistance of GFRP bars and the high elastic modulus of steel reinforcement, the hybrid-reinforced beams reduced the influence of corrosion damage at the outer tensile reinforcement layer on load capacity degradation while maintaining favorable deformation compatibility. This indicates that hybrid reinforcement can effectively mitigate the adverse effects of steel corrosion and interfacial degradation on the flexural capacity of beams in salt-corrosion environments. Based on material degradation coefficients, sectional strain compatibility, and internal force equilibrium, a prediction model for the residual flexural capacity of hybrid-reinforced beams under coupled exposure was developed. The predicted values agreed with the test results within an overall error of ±15%. The findings provide a useful reference for durability design and residual capacity assessment of hybrid-reinforced concrete beams in aggressive salt environments.